Radio transceivers typically include a frequency mixer which converts a signal from a baseband to a radio frequency (RF) band or vice versa. The frequency mixer upconverts a transmission signal from the baseband to the RF band in transmission and/or downconverts a received signal from the RF band to the baseband in reception. Alternatively, the received RF signal may be converted to an intermediate frequency band in some implementations.
The frequency mixer has a signal to be mixed, i.e. upconverted or downconverted, and one or more local oscillator signals as input signals, and it produces an output signal at a frequency which is a linear combination of the frequencies of the input signals. Typically, the local oscillator signals input to the frequency mixer are the same signal but with different phase shifts.
The mixer may be either a passive mixer or an active mixer. Passive mixers have no energy source but an input signal and a local oscillator signal. Accordingly, the output power may not be greater than the input power. Active mixers, on the other hand, require an additional energy source in order to amplify the input signal. Accordingly, the output power may be higher than the input power.
An advantage the active mixer provides over the passive mixer is that the active mixer amplifies the signal being mixed. As a result, the power of the resulting signal is higher when the active mixer is utilized. On the other hand, the amplification results in an increased noise power, too. Additionally, linearity properties of the active mixers are generally quite poor, and the active mixers consume power which may be a limited resource in some implementations.
On the contrary, passive mixers have typically good linearity and noise properties and they do not consume power. Their only drawback is that they attenuate the signal being mixed instead of amplifying it. The level of the attenuation depends on the implementation.
FIG. 1 illustrates a prior-art receiver structure which converts a received radio signal directly to the baseband. The receiver comprises a first amplifier 2 before mixers 4 and 5. The amplifier 2 is typically a low-noise amplifier. Bandpass filters 1 and 3 have been provided before and after the amplifier 2 to remove undesired frequency components. Mixers 4 and 5 mix in-phase (I) and quadrature (Q) components of the received radio signal to the baseband with local oscillator signals LO_0, LO_90, LO_180, and LO_270. The number refers to the phase shift of the respective local oscillator signal. After the downmixing, baseband amplifiers 6 and 7 amplify the downmixed I and Q components, respectively, and low-pass filters 8 and 9 remove harmonic signal components resulting from the downmixing. Amplifiers 10 and 11 further amplify the low-pass filtered signals before analog-to-digital (A/D) conversion in an A/D-converter 12.
Typically, noise figures of the baseband amplifiers 6 and 7 are relatively poor due to flicker noise (known also as 1/f noise), among others. Therefore, the signal levels obtained from the mixers 4 and 5 have to be higher than the noise level of the baseband amplifiers 6 and 7. If the mixers 6 and 7 are passive mixers, the total noise figure of the receiver may be improved only in the first amplifier 2 before the mixers 4 and 5. In this case, the level of the output signal of the amplifier 2 may rise so high that good linearity properties of the passive mixers are wasted. As a consequence, the advantages of the passive mixers over the active mixers are also wasted.
Another problem related to the passive mixers is that designing a low-pass filter following a passive mixer may be difficult. The low-pass filter should be located before the first baseband amplifier in order to prevent cross-modulation and inter-modulation caused by undesired strong signal components. In CMOS implementations, it is hard to define the corner frequency of the filter accurately, since an output impedance of the mixer, the pulse ratio of a local oscillator and the tolerances of capacitance values of the mixer components affect the corner frequency. Thus, the filter is typically arranged to follow the first baseband amplifier, which results in performance close to that of an active mixer. Accordingly, the advantages of the passive mixer are wasted again.